1. Field of the Invention
The present invention relates to a system for positioning a controlled object such as table or bite in a machine tool of a closed loop type numerically controlled apparatus. More particularly the present invention relates to a system for returning the controlled object to an original position.
2. Description of the Prior Art
In conventional machine tools which are numerically controlled, various position detectors such as resolver, Inductosyn (trademark) and rotary encoder are used for the purpose of detecting the position of the table. Herein, a rotary encoder, for example, can be inexpensively used in the numerically controlled machine tool. Therefore, the conventional machine which uses a rotary encoder becomes inexpensive and it can be used in a manner of digital control because the rotary encoder generates pulse outputs. However, in the conventional numerically controlled machine tool which uses a rotary encoder as the position detector, the time delay between command pulses and feedback pulses varies in accordance with the variation of the load, so that the table can not be precisely returned to the original position. Further, in such a conventional numerically controlled machine tool provided with a rotary encoder, a positioning by grid point control as applicable to a numerically controlled machine tool using a resolver or Inductosyn, or an electronical displacement of the grid points can not be expected.